JP5608520B2 - Method for manufacturing transistor - Google Patents

Description

  The technical field relates to a thin film transistor using an oxide semiconductor.

  In recent years, metal oxides that exhibit semiconductor characteristics, called oxide semiconductors, have attracted attention as a new semiconductor material that combines high mobility obtained from polysilicon and uniform element characteristics obtained from amorphous silicon. For example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like can be given as metal oxides that exhibit semiconductor characteristics.

  Patent Documents 1 and 2 propose a thin film transistor in which a metal oxide exhibiting semiconductor characteristics is used for a channel formation region.

JP 2007-123861 A JP 2007-96055 A

  It is an object to provide a thin film transistor using an oxide semiconductor with favorable electrical characteristics.

  One embodiment of the present invention includes a gate electrode provided over a substrate, a gate insulating film provided over the gate electrode, an oxide semiconductor film provided over the gate electrode and the gate insulating film, and an oxide semiconductor film. A metal oxide film provided on the metal oxide film; the oxide semiconductor film is in contact with the metal oxide film; and the other oxide semiconductor film A thin film transistor having a region (metal high concentration region) having a metal concentration higher than that of the region.

  The metal contained in the oxide semiconductor film may exist as crystal grains or microcrystals in the high metal concentration region.

  One embodiment of the present invention includes a gate electrode provided over a substrate, a gate insulating film provided over the gate electrode, and an oxide containing indium, gallium, and zinc provided over the gate electrode and the gate insulating film. A semiconductor film, a titanium oxide film provided over the oxide semiconductor film, and a titanium film provided over the titanium oxide film, the oxide semiconductor film being in contact with the titanium oxide film and being an oxide The thin film transistor includes a region having a higher concentration of indium than other regions of the semiconductor film.

  Indium may exist as crystal grains or microcrystals in a region where the concentration of indium is higher than that of other regions of the oxide semiconductor film.

  A thin film transistor using an oxide semiconductor with favorable electrical characteristics can be provided.

FIG. 10 is a schematic cross-sectional view of a thin film transistor using an oxide semiconductor. FIG. 6 is an energy band diagram between a source electrode and a drain electrode in a thin film transistor using an oxide semiconductor. FIG. 10 shows a crystal structure of a metal and oxygen in an In—Ga—Zn—O-based oxide semiconductor. The figure which shows a structural model. The figure which shows a structural model. The figure which shows a structural model. (A) The graph which shows the CV characteristic of the sample 1, (B) The graph which shows the relationship between the gate voltage (Vg) of the sample 1, and (1 / C) ² . (A) The graph which shows the CV characteristic of the sample 2, (B) The graph which shows the relationship between the gate voltage (Vg) of the sample 2, and (1 / C) ² . The figure which shows the crystal structure of the titanium dioxide which has a rutile structure. The state density figure of the titanium dioxide which has a rutile structure. The state density figure of the titanium dioxide which has a rutile structure of an oxygen deficient state. The state density diagram of titanium monoxide. FIG. 14 illustrates an example of an electronic device to which a thin film transistor is applied. A cross-sectional TEM photograph of a thin film transistor using an In—Ga—Zn—O-based oxide semiconductor.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following description, and it is easily understood by those skilled in the art that modes and details can be variously changed without departing from the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments below.

(Embodiment 1)
FIG. 1A is a schematic cross-sectional view of a thin film transistor using an oxide semiconductor. The thin film transistor includes a substrate 10, a gate electrode 20, a gate insulating film 30, an oxide semiconductor film 40, a metal oxide film 60, a metal film 70, and an insulating film 80.

  The thin film transistor illustrated in FIG. 1A is a bottom-gate type with a channel etch structure. However, the structure of the thin film transistor is not limited to this, and an arbitrary top gate type or bottom gate type can be used.

As the substrate 10, a substrate having an insulating surface is used. It is appropriate to use a glass substrate as the substrate 10. When the temperature of the subsequent heat treatment is high, a glass substrate having a strain point of 730 ° C. or higher is preferably used. In view of heat resistance, a glass substrate containing more barium oxide (BaO) than boron oxide (B ₂ O ₃ ) is preferable.

  In addition to the glass substrate, a substrate made of an insulator such as a ceramic substrate, a quartz glass substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 10. In addition, a crystallized glass substrate or the like may be used as the substrate 10.

  Further, an insulating film serving as a base film may be provided between the substrate 10 and the gate electrode 20. The base film has a function of preventing diffusion of impurity elements from the substrate 10. Note that the insulating film serving as a base film may be formed using one or a plurality of films selected from a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film.

  As the gate electrode 20, a metal conductive film can be used. As a material of the metal conductive film, an element selected from aluminum (Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), and tungsten (W), or An alloy containing these elements as a main component can be used. For example, a three-layer structure of titanium film-aluminum film-titanium film or a three-layer structure of molybdenum film-aluminum film-molybdenum film can be used as the metal conductive film. Note that the metal conductive film is not limited to a three-layer structure, and may have a single layer structure, a two-layer structure, or a stacked structure including four or more layers.

  As the gate insulating film 30, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, a hafnium oxide film, or the like is used. Can do.

As an oxide semiconductor used for the oxide semiconductor film 40, an In—Sn—Ga—Zn—O-based oxide semiconductor that is a quinary metal oxide, or an In—Ga—Zn— that is a quaternary metal oxide. O-based oxide semiconductor, In-Sn-Zn-O-based oxide semiconductor, In-Al-Zn-O-based oxide semiconductor, Sn-Ga-Zn-O-based oxide semiconductor, Al-Ga-Zn-O-based Oxide semiconductors, Sn-Al-Zn-O-based oxide semiconductors, In-Zn-O-based oxide semiconductors that are ternary metal oxides, Sn-Zn-O-based oxide semiconductors, Al-Zn-O Oxide semiconductor, Zn—Mg—O oxide semiconductor, Sn—Mg—O oxide semiconductor, In—Mg—O oxide semiconductor, In—Ga—O oxide semiconductor, binary metal In-O-based oxide semiconductor, Sn-O-based oxide semiconductor, Zn- Or the like can be used system oxide semiconductor. Note that in this specification, for example, an In—Sn—Ga—Zn—O-based oxide semiconductor is a metal oxide containing indium (In), tin (Sn), gallium (Ga), and zinc (Zn). The composition ratio is not particularly limited. The oxide semiconductor film 40 may include silicon oxide (SiO ₂ ).

For the oxide semiconductor film 40, an oxide semiconductor having a structure represented by InMO ₃ (ZnO) _m (m> 0) can be used. Here, M represents one or more metal elements selected from gallium (Ga), aluminum (Al), manganese (Mn), and cobalt (Co). Examples corresponding to M include gallium alone, gallium and aluminum, gallium and manganese, gallium and cobalt, and the like.

Note that among oxide semiconductors having a structure represented by InMO ₃ (ZnO) _m (m> 0), an oxide semiconductor including gallium (Ga) as M is an In—Ga—Zn—O-based oxide. Also referred to as a physical semiconductor.

  The oxide semiconductor film 40 intentionally excludes impurities such as hydrogen, moisture, hydroxyl groups, and hydroxides (also referred to as hydrogen compounds), which are causes of donors, and then decreases oxygen in the exclusion process of these impurities. By supplying this, it is highly purified and electrically i-type (intrinsic). This is to suppress fluctuations in the electrical characteristics of the thin film transistor.

The smaller the hydrogen in the oxide semiconductor film 40 is, the closer the oxide semiconductor film 40 is to i-type. Therefore, the concentration of hydrogen contained in the oxide semiconductor film 40 is 5 × 10 ¹⁹ / cm ³ or less, preferably 5 × 10 ¹⁸ / cm ³ or less, more preferably 5 × 10 ¹⁷ / cm ³ or less, and still more preferably It may be less than 5 × 10 ¹⁶ / cm ³ . The concentration of the hydrogen can be measured by secondary ion mass spectrometry (SIMS; Secondary Ion Mass Spectrometry).

By removing hydrogen contained in the oxide semiconductor film 40 as much as possible, the carrier density in the oxide semiconductor film 40 is less than 5 × 10 ¹⁴ / cm ³ , preferably 5 × 10 ¹² / cm ³ or less, more preferably 5 × 10 ¹⁰ / cm ³ or less. The carrier density of the oxide semiconductor film 40 can be obtained by fabricating a MOS capacitor using the oxide semiconductor film 40 and evaluating the CV measurement result (CV characteristics) of the MOS capacitor.

  The oxide semiconductor is a wide gap semiconductor. For example, the band gap of silicon is 1.12 eV, whereas the band gap of an In—Ga—Zn—O-based oxide semiconductor is 3.15 eV.

  An oxide semiconductor that is a wide gap semiconductor has a low minority carrier density and is less likely to induce minority carriers. Therefore, in the thin film transistor using the oxide semiconductor film 40, it can be said that a tunnel current hardly occurs and an off current hardly flows. Therefore, an off current per channel width of 1 μm of the thin film transistor using the oxide semiconductor film 40 can be 100 aA / μm or less, preferably 10 aA / μm or less, more preferably 1 aA / μm or less.

  In addition, since the oxide semiconductor is a wide gap semiconductor, impact ionization and avalanche breakdown hardly occur in the thin film transistor using the oxide semiconductor film 40. Therefore, it can be said that the thin film transistor using the oxide semiconductor film 40 has resistance to hot carrier deterioration. This is because hot carrier deterioration is mainly caused by carriers increasing due to avalanche breakdown, and carriers accelerated at high speed are injected into the gate insulating film.

  The metal film 70 is used as a source electrode or a drain electrode. As the metal film 70, a metal material such as aluminum (Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), or these metals An alloy material containing the material as a main component can be used. The metal film 70 is formed of chromium (Cr), tantalum (Ta), titanium (Ti), molybdenum (Mo) on one surface or both surfaces of a metal film using aluminum (Al), copper (Cu), or the like. ), Refractory metal films using tungsten (W) or the like may be stacked. Aluminum such as silicon (Si), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), scandium (Sc), yttrium (Y), etc. By using aluminum to which an element for preventing generation of hillocks and whiskers generated in the film is used as a material, the metal film 70 having excellent heat resistance can be obtained.

  As the metal oxide film 60, a film containing a metal oxide contained in the metal film 70 can be used. For example, when the metal film 70 is a film containing titanium, a titanium oxide film or the like can be used as the metal oxide film 60.

  The oxide semiconductor film 40 has a region in contact with the metal oxide film 60 and having a metal concentration higher than that of the other regions of the oxide semiconductor film 40. The region where the metal concentration is high is also referred to as a metal high concentration region 50.

  FIG. 1B is a schematic cross-sectional view in which the region 100 in FIG.

  As shown in FIG. 1B, the metal contained in the oxide semiconductor film 40 may exist as crystal grains or microcrystals in the metal high concentration region 50.

  FIG. 2 is an energy band diagram (schematic diagram) between a source electrode and a drain electrode in the thin film transistor having the configuration shown in FIG. This figure assumes a case where the potential difference between the source electrode and the drain electrode is zero.

  Here, the metal high concentration region 50 is treated as a metal. The oxide semiconductor film 40 is highly purified and electrically i-type (intrinsic) by removing impurities as much as possible and supplying oxygen. As a result, in the energy band diagram, the Fermi level (Ef) is in the vicinity of the center of the band gap inside the oxide semiconductor film 40.

  From this energy band diagram, it can be seen that in the oxide semiconductor film 40, there is no barrier at the interface between the high-concentration metal region 50 and another region, and a good contact is obtained. The same applies to the interface between the metal high concentration region 50 and the metal oxide film 60 and the interface between the metal oxide film 60 and the metal film 70.

(Embodiment 2)
A manufacturing process of the thin film transistor having the structure illustrated in FIGS.

  First, after a conductive film is formed over the substrate 10 having an insulating surface, the gate electrode 20 is formed by a first photolithography process.

  The resist mask used for the first photolithography process may be formed by an inkjet method. When the resist mask is formed by an ink-jet method, a manufacturing cost can be reduced because a photomask is not used.

  Next, a gate insulating film 30 is formed on the gate electrode 20.

  The gate insulating film 30 is formed by a method such as a plasma CVD method or a sputtering method. As the gate insulating film 30, a film using silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, hafnium oxide, or the like is preferable.

  The gate insulating film 30 in contact with the oxide semiconductor film 40 is desirably a dense film with high withstand voltage. Therefore, it is particularly suitable to use as the gate insulating film 30 a dense film having a high withstand voltage that is formed by a high-density plasma CVD method using μ waves (2.45 GHz).

  Interfacial characteristics between the gate insulating film 30 which is a dense film having a high withstand voltage obtained in this way and the oxide semiconductor film 40 which is made i-type by removing impurities as much as possible and supplying oxygen are good. Become.

If the interface characteristics between the oxide semiconductor film 40 and the gate insulating film 30 are poor, impurities in the gate bias / thermal stress test (BT test: 85 ° C., 2 × 10 ⁶ V / cm, 12 hours) As a result, the bond between the main component of the oxide semiconductor and the main component of the oxide semiconductor is broken, and a shift of the threshold voltage is induced by the generated dangling bonds.

The gate insulating film 30 may have a stacked structure of a nitride insulating film and an oxide insulating film. For example, after a silicon nitride film (SiN _y (y> 0)) having a thickness of 50 nm to 200 nm is formed as the first gate insulating film by a sputtering method, the second gate insulating film is formed on the first gate insulating film. As a gate insulating film 30 having a stacked structure can be formed by forming a silicon oxide film (SiO _x (x> 0)) with a thickness of 5 nm to 300 nm. The thickness of the gate insulating film 30 may be set as appropriate depending on characteristics required for the thin film transistor, and may be about 350 nm to 400 nm.

  Preferably, as a pretreatment for forming the gate insulating film 30, impurities such as hydrogen and moisture adsorbed on the substrate 10 are preliminarily heated in the preheating chamber of the sputtering apparatus by preheating the substrate 10 on which the gate electrode 20 is formed. Desorption and evacuation are good. This is to prevent impurities such as hydrogen and moisture from being contained as much as possible in the gate insulating film 30 and the oxide semiconductor film 40 formed thereafter. Alternatively, the substrate 10 may be preheated when the gate insulating film 30 is formed on the substrate 10.

  The preheating temperature is suitably 100 ° C. or higher and 400 ° C. or lower. If it is 150 degreeC or more and 300 degrees C or less, it is still more suitable. In addition, a cryopump is appropriate as the exhaust means in the preheating chamber.

  Next, the oxide semiconductor film 40 is formed over the gate insulating film 30. An appropriate thickness of the oxide semiconductor film 40 is 2 nm to 200 nm.

  The oxide semiconductor film 40 is formed by a sputtering method. Film formation by a sputtering method is performed in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen.

As a target used for forming the oxide semiconductor film 40 by a sputtering method, a metal oxide containing zinc oxide as a main component can be used. The composition ratios of In ₂ O ₃ : Ga ₂ O ₃ : ZnO = 1: 1: 1 [mol%] or In: Ga: Zn = 1: 1: 0.5 [atom%], In : Ga: Zn = 1: 1: 1 [atom%] or In: Ga: Zn = 1: 1: 2 [atom%], indium (In), gallium (Ga), and zinc (Zn) An oxide semiconductor target for film formation containing can also be used. In addition, the filling rate of the oxide semiconductor deposition target is appropriately 90% to 100%. If it is 95% or more and 99.9% or less, it is more preferable. This is because a denser oxide semiconductor film can be formed when an oxide semiconductor deposition target with a high filling rate is used.

  Before the oxide semiconductor film 40 is formed, the substrate 10 is held in a processing chamber in a reduced pressure state, and the substrate 10 is heated to a temperature of room temperature to less than 400 ° C. Thereafter, a voltage is applied between the substrate 10 and the target while removing residual moisture in the processing chamber and introducing a sputtering gas from which hydrogen and moisture have been removed, whereby the oxide semiconductor film 40 is formed on the substrate 10. Is deposited.

It is appropriate to use an adsorption-type vacuum pump as an exhausting means for removing residual moisture in the processing chamber. Examples include a cryopump, an ion pump, and a titanium sublimation pump. Moreover, what added the cold trap to the turbo pump can also be used as an exhaust means. A film containing a hydrogen atom, a hydrogen molecule, a compound containing a hydrogen atom such as water (H ₂ O), or the like (preferably together with a compound containing a carbon atom) is exhausted from the treatment chamber to form a film in the treatment chamber. The concentration of impurities contained in the oxide semiconductor film 40 can be reduced. In addition, the temperature of the substrate 10 when the oxide semiconductor film 40 is formed is set to be room temperature or higher and lower than 400 ° C. by performing film formation by a sputtering method while removing residual moisture in the treatment chamber with a cryopump. it can.

  Note that dust attached to the surface of the gate insulating film 30 is preferably removed by reverse sputtering before the oxide semiconductor film 40 is formed by a sputtering method. Reverse sputtering is a method of cleaning the substrate surface with reactive plasma generated by applying a voltage to the substrate side using an RF power source without applying a voltage to the target side. Note that reverse sputtering is performed in an argon atmosphere. Further, nitrogen, helium, oxygen, or the like may be used instead of argon.

After the oxide semiconductor film 40 is formed, the oxide semiconductor film 40 is dehydrated or dehydrogenated. The temperature of the heat treatment for dehydration or dehydrogenation is suitably 400 ° C. or higher and 750 ° C. or lower, and particularly preferably 425 ° C. or higher. Note that the heat treatment time may be 1 hour or less if the temperature of the heat treatment is 425 ° C. or higher, but is preferably longer than 1 hour if the temperature is less than 425 ° C. In this specification, the desorption of hydrogen molecules (H ₂ ) by this heat treatment is not only called dehydrogenation, but desorption of hydrogen atoms (H), hydroxyl groups (OH), and the like. Including the dehydration or dehydrogenation for convenience.

For example, the substrate 10 over which the oxide semiconductor film 40 is formed is introduced into an electric furnace which is one of heat treatment apparatuses, and heat treatment is performed in a nitrogen atmosphere. Thereafter, high purity oxygen gas, high purity dinitrogen monoxide (N ₂ O) gas, or ultra-dry air (with a dew point of −40 ° C. or lower, preferably −60 ° C. or lower, nitrogen and oxygen of 4 in the same furnace) Cooling is performed by introducing a gas mixed at a ratio of 1: 2. The oxygen gas or dinitrogen monoxide (N ₂ O) gas preferably does not contain water or hydrogen. The purity of oxygen gas or dinitrogen monoxide (N ₂ O) gas is 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (that is, oxygen gas or dinitrogen monoxide (N ₂ )). O) It is appropriate that the impurity concentration in the gas is 1 ppm or less, preferably 0.1 ppm or less.

  Note that the heat treatment apparatus is not limited to an electric furnace, and for example, an RTA (Rapid Thermal Annial) apparatus such as a GRTA (Gas Rapid Thermal Anneal) apparatus or an LRTA (Lamp Rapid Thermal Anneal) apparatus can be used.

  Further, the heat treatment for dehydration or dehydrogenation of the oxide semiconductor film 40 is performed on the oxide semiconductor film 40 before or after the oxide semiconductor film 40 is processed into an island shape by the second photolithography process. Can be done against.

  Through the above steps, the entire oxide semiconductor film 40 is brought into an oxygen-excess state, whereby the entire oxide semiconductor film 40 is increased in resistance, that is, i-type.

  Next, a metal film 70 is formed over the gate insulating film 30 and the oxide semiconductor film 40. The metal film 70 may be formed by a sputtering method, a vacuum evaporation method, or the like. Further, the metal film 70 may have a single layer structure or a laminated structure of two or more layers.

  Thereafter, a resist mask is formed over the metal film 70 by a third photolithography process, and selective etching is performed to form a source electrode and a drain electrode, and then the resist mask is removed.

  The channel length of the thin film transistor is determined by the distance between the lower end of the source electrode adjacent to the lower end of the drain electrode on the oxide semiconductor film 40. That is, it can be said that the channel length of the thin film transistor is determined by the exposure condition when forming the resist mask in the third photolithography process. Ultraviolet, KrF laser light, or ArF laser light can be used for light exposure for forming the resist mask in the third photolithography process. In addition, when the channel length is less than 25 nm, exposure may be performed using extreme ultraviolet (Extreme Ultraviolet) with a very short wavelength of several nm to several tens of nm. This is because the exposure with extreme ultraviolet rays has a high resolution and a large depth of focus. Therefore, the channel length of the thin film transistor can be 10 nm to 1000 nm depending on the type of light used for exposure.

  Note that the material of the metal film 70, the material of the oxide semiconductor film 40, and the etching conditions need to be adjusted as appropriate so that the oxide semiconductor film 40 is not removed when the metal film 70 is etched.

  As an example, in the case where a titanium film is used as the metal film 70 and an In—Ga—Zn—O-based oxide semiconductor film is used as the oxide semiconductor film 40, ammonia overwater (ammonia, water, And a mixed solution of hydrogen peroxide water).

  Note that in the third photolithography process, only part of the oxide semiconductor film 40 may be etched, whereby the oxide semiconductor film 40 having a groove (a depressed portion) may be formed. Further, the resist mask for forming the source electrode and the drain electrode may be formed by an inkjet method. When the resist mask is formed by an ink-jet method, a manufacturing cost can be reduced because a photomask is not used.

After forming the source electrode and the drain electrode, the exposed oxide semiconductor film 40 is exposed by plasma treatment using a gas such as dinitrogen monoxide (N ₂ O), nitrogen (N ₂ ), or argon (Ar). You may remove the water (adsorbed water) etc. which adhered to the surface. In the plasma treatment, a mixed gas of oxygen and argon can be used.

  In the case where plasma treatment is performed, the insulating film 80 that is in contact with part of the oxide semiconductor film 40 is formed without being exposed to the air as it is. In the thin film transistor illustrated in FIG. 1, the oxide semiconductor film 40 and the insulating film 80 are in contact with each other in a region where the oxide semiconductor film 40 and the metal film 70 do not overlap.

  As an example of the insulating film 80, the substrate 10 on which the oxide semiconductor film 40 and the metal film 70 are formed is heated at a temperature of room temperature to less than 100 ° C., and then a sputtering gas containing high-purity oxygen from which hydrogen and moisture are removed. And a silicon oxide film containing defects formed using a silicon target.

  The insulating film 80 is preferably formed while removing residual moisture in the processing chamber. This is for preventing hydrogen, a hydroxyl group, and moisture from being contained in the oxide semiconductor film 40 and the insulating film 80.

It is appropriate to use an adsorption-type vacuum pump as an exhausting means for removing residual moisture in the processing chamber. Examples include a cryopump, an ion pump, and a titanium sublimation pump. Moreover, what added the cold trap to the turbo pump can also be used as an exhaust means. By exhausting hydrogen atoms, hydrogen molecules, compounds containing hydrogen atoms such as water (H ₂ O), and the like from the treatment chamber, the concentration of impurities contained in the insulating film 80 formed in the treatment chamber can be reduced. .

  Note that as the insulating film 80, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or the like can be used in addition to the silicon oxide film.

  After the insulating film 80 is formed, heat treatment is performed at 100 ° C. to 400 ° C., preferably 150 ° C. to less than 350 ° C. in an inert gas atmosphere or a nitrogen gas atmosphere. When heat treatment is performed, impurities such as hydrogen, moisture, a hydroxyl group, and hydride contained in the oxide semiconductor film 40 diffuse into the insulating film 80 including defects. As a result, impurities contained in the oxide semiconductor film 40 can be further reduced.

  Further, by the heat treatment, a metal oxide film 60 is formed at the interface between the oxide semiconductor film 40 and the metal film 70, and the metal high-concentration region 50 is formed in a region in contact with the metal oxide film 60 in the oxide semiconductor film 40. Is formed.

  Note that the metal oxide film 60 may be formed over the oxide semiconductor film 40 by a sputtering method or the like before the metal film 70 is formed. In this case, the thin film transistor of FIG. 1 can be obtained by removing the metal oxide film 60 provided in a region where the oxide semiconductor film 40 and the metal film 70 do not overlap.

  The heat treatment may be performed before the insulating film 80 is formed.

  Through the above steps, the thin film transistor having the structure illustrated in FIG. 1 can be formed.

(Embodiment 3)
In the thin film transistor having the structure shown in FIG. 1, a metal oxide film 60 is formed at the interface between the oxide semiconductor film 40 and the metal film 70, and a high metal concentration is present in a region in contact with the metal oxide film 60 in the oxide semiconductor film 40. The result of having verified by the computational science about the phenomenon in which the area | region 50 is formed is shown.

  In the following calculation, the case where the oxide semiconductor film 40 is an In—Ga—Zn—O-based oxide semiconductor film was considered. The metal film 70 is considered to be a tungsten (W) film, a molybdenum (Mo) film, or a titanium (Ti) film.

[Phenomenon in which metal high concentration region 50 is formed]
The energy (defect formation energy E _def ) required for each of the oxides of indium, gallium, and zinc constituting the In—Ga—Zn—O-based oxide semiconductor to form an oxygen-deficient state was calculated.

The defect formation energy E _def is defined by the following formula (1).

Note that E (A _m O _n-1 ) is the energy of the oxide A _m O _n-1 having oxygen vacancies, E (O) is the energy of oxygen atoms, and E (A _m O _n ) is the oxidation without oxygen vacancies. It represents the energy of the object _a m _{O n.} A represents indium alone, gallium alone, zinc alone, or indium, gallium and zinc.

Further, the relationship between the oxygen deficiency concentration n and the deficiency formation energy E _def is approximately expressed by the following equation (2).

N represents the number of oxygen in a state where no defect is formed, k _B represents a Boltzmann constant, and T represents an absolute temperature.

From the mathematical formula (2), it is found that as the deficiency formation energy E _def increases, the oxygen deficiency concentration n, that is, the amount of oxygen deficiency decreases.

CASTEP, which is a program of the density functional method, was used for the calculation of the defect formation energy E _def . A plane wave basis pseudopotential method was used as the density functional method, and GGA-PBE was used as the functional. The cut-off energy was 500 eV. The number of grids at point k is 3 × 3 × 1 for an oxide containing indium, gallium, and zinc (hereinafter also referred to as “IGZO”), and is also referred to as an oxide of indium (hereinafter referred to as “In ₂ O ₃ ”). ) Is 2 × 2 × 2, gallium oxide (hereinafter also referred to as “Ga ₂ O ₃ ”) is 2 × 3 × 2, and zinc oxide (hereinafter also referred to as “ZnO”). 4 × 4 × 1.

The crystal structure of Ga and Zn is higher than that of 84 atoms obtained by doubling the structure of symmetry R-3 (international number: 148) for the IGZO to the a-axis and b-axis, respectively. A structure arranged so as to be minimized was used. For In ₂ O ₃ , an 80 atom bixbyte structure was used, for Ga ₂ O ₃ , an 80 atom β-Gallia structure, and for ZnO, an 80 atom Wurtz structure was used.

Table 1 shows the defect formation energy E _def when A is indium alone, gallium alone, zinc alone, indium, gallium, and zinc in Formula (1), respectively. FIG. 3 shows a crystal structure of metal and oxygen in the In—Ga—Zn—O-based oxide semiconductor.

The deficiency formation energy E _def of IGZO (Model 1) is obtained when oxygen is adjacent to three indium atoms and one zinc atom in the IGZO crystal when A is indium, gallium, and zinc (see FIG. 3A). It corresponds to the defect formation energy.

The defect formation energy E _def of IGZO (Model 2) is the amount of oxygen adjacent to three indium atoms and one gallium atom in the IGZO crystal when A is indium, gallium, and zinc (see FIG. 3B). It corresponds to the defect formation energy.

The deficiency formation energy E _def of IGZO (Model 3) is obtained when oxygen is adjacent to two zinc atoms and two gallium atoms in the IGZO crystal when A is indium, gallium, and zinc (see FIG. 3C). It corresponds to the defect formation energy.

The higher the defect formation energy E _def , the higher the energy required to form the oxygen deficiency state. That is, the larger the defect formation energy E _{def, the} stronger the bond between oxygen and metal. In other words, from Table 1, it can be said that indium having the smallest defect formation energy E _def has the weakest bond with oxygen.

  The oxygen deficiency state in the In—Ga—Zn—O-based oxide semiconductor occurred because the metal film 70 used as the source electrode or the drain electrode extracted oxygen from the oxide semiconductor film 40. A part of the oxide semiconductor film 40 in the oxygen deficient state thus becomes the metal high concentration region 50. Depending on the presence or absence of the metal high concentration region 50, the carrier density of the oxide semiconductor film 40 differs by at least two orders of magnitude. This is because oxygen is extracted from the oxide semiconductor film 40 so that the oxide semiconductor film 40 is n-type. The n-type means that the number of electrons that are majority carriers increases.

[Phenomenon in which metal oxide film 60 is formed]
Quantum molecular dynamics (QMD) calculation was performed on the stacked structure of the oxide semiconductor film 40 using the In—Ga—Zn—O-based oxide semiconductor and the metal film 70. This is for confirming extraction of oxygen from the oxide semiconductor by a metal.

  The structure to be calculated was prepared as follows. First, structural optimization is performed on an amorphous In—Ga—Zn—O-based oxide semiconductor (hereinafter also referred to as “a-IGZO”) manufactured by a classical molecular dynamics (CMD) method by a QMD method. It was. Furthermore, a metal film having a crystal of metal atoms (W, Mo, Ti) was stacked on the a-IGZO film obtained by cutting the unit cell whose structure was optimized. And the structure optimization was performed with respect to the produced structure. Using this structure as a starting point, calculations were performed at 623.0 K using the QMD method. In order to estimate only the interface interaction, the lower end of the a-IGZO film and the upper end of the metal film were fixed.

The calculation conditions for CMD calculation are shown below. For the calculation program, Materials Explorer was used. a-IGZO was produced under the following conditions. In a calculation cell having a side of 1 nm, all 84 atoms were randomly arranged at a ratio of In: Ga: Zn: O = 1: 1: 1: 4, and the density was set to 5.9 g / cm ³ . The CMD calculation was performed with an NVT ensemble. After the temperature was gradually lowered from 5500K to 1K, the structure was relaxed for 10 ns at 1K. The time increment was 0.1 fs and the total calculation time was 10 ns. For the potential, the Born-Mayer-Huggins type was applied between the metal and oxygen and between the oxygen and oxygen, and the Lennard-Jones type was applied between the metal and metal. The charges were In: +3, Ga: +3, Zn: +2, and O: -2.

  The calculation conditions for QMD calculation are shown below. First-principles calculation software CASTEP was used as the calculation program. The functional was GGA-PBE. For the pseudopotential, Ultrasoft was used. The cut-off energy was 260 eV, and the number of k points was 1 × 1 × 1. QMD calculation was performed with an NVT ensemble, and the temperature was 623K. The time increment was 1.0 fs and the total calculation time was 2.0 ps.

  The results of the above calculation will be described using the structural models shown in FIGS. 4 to 6, white circles represent crystal metal atoms contained in the metal film stacked on the a-IGZO film, and black circles represent oxygen atoms.

  FIG. 4 shows a structural model in the case where a metal film having a tungsten (W) crystal is stacked on the a-IGZO film. 4A corresponds to the structure before the QMD calculation, and FIG. 4B corresponds to the structure after the QMD calculation.

  FIG. 5 shows a structural model in the case where a metal film having a molybdenum (Mo) crystal is stacked over an a-IGZO film. 5A corresponds to the structure before the QMD calculation, and FIG. 5B corresponds to the structure after the QMD calculation.

  FIG. 6 shows a structural model when a metal film having a titanium (Ti) crystal is stacked on an a-IGZO film. 6A corresponds to the structure before the QMD calculation, and FIG. 6B corresponds to the structure after the QMD calculation.

  5A and 6A, when a metal film having molybdenum or titanium crystals is stacked on the a-IGZO film, oxygen atoms have already moved to the metal film before the structure optimization. I found out. 4B, FIG. 5B, and FIG. 6B, when a metal film having a titanium crystal is stacked on the a-IGZO film, oxygen atoms are the most in the metal film. I found that it was moving a lot. Therefore, when titanium is used as a metal, oxygen is most likely to be extracted from the oxide semiconductor by the metal. Thus, the most suitable electrode for causing oxygen deficiency in the a-IGZO film was a metal film having titanium crystals.

[Carrier density in oxide semiconductor film 40]
Regarding the extraction of oxygen from the oxide semiconductor film 40 by the metal contained in the metal film 70, an element was actually fabricated and evaluated. Specifically, in the oxide semiconductor film 40 when a metal film having an effect of extracting oxygen is stacked on the oxide semiconductor film, and when a metal film having no effect of extracting oxygen is stacked on the oxide semiconductor film. The carrier density was calculated and the results were compared.

  The carrier density in the oxide semiconductor film can be obtained by fabricating a MOS capacitor using the oxide semiconductor film and evaluating the CV measurement result (CV characteristics) of the MOS capacitor. .

The carrier density was measured by the following procedures (1) to (3). (1) A CV characteristic in which the relationship between the gate voltage (Vg) and the capacitance (C) of the MOS capacitor is plotted is acquired. (2) A graph representing the relationship between the gate voltage (Vg) and (1 / C) ² is acquired from the CV characteristics, and the differential value of (1 / C) ² in the weak inversion region in the graph is obtained. Ask. (3) The obtained differential value is substituted into the following formula (3) representing the carrier density (Nd).

Note that e represents the amount of electricity, ε ₀ represents the dielectric constant of vacuum, and ε represents the relative dielectric constant of the oxide semiconductor.

  As a sample for measurement, a MOS capacitor using a metal film having an effect of extracting oxygen (hereinafter also referred to as “sample 1”) and a MOS capacitor using a metal film having no effect of extracting oxygen (hereinafter referred to as “sample”). Also referred to as “Sample 2”). Note that a titanium film was applied as a metal film having an effect of extracting oxygen. In addition, as a metal film having no effect of extracting oxygen, a titanium film and a film having a titanium nitride film on the surface (oxide semiconductor film side) were applied.

The details of the sample are as follows.
Sample 1:
An oxide semiconductor film having a thickness of 2 μm using a titanium film having a thickness of 400 nm on a glass substrate and using an In—Ga—Zn—O-based oxide semiconductor (a-IGZO) having an amorphous structure on the titanium film. A silicon oxynitride film with a thickness of 300 nm is formed over the oxide semiconductor film, and a silver film with a thickness of 300 nm is formed over the silicon oxynitride film.
Sample 2:
An In—Ga—Zn—O-based oxide semiconductor having a 300 nm thick titanium film over a glass substrate, a 100 nm thick titanium nitride film over the titanium film, and an amorphous structure over the titanium nitride film A 2 μm-thick oxide semiconductor film using (a-IGZO), a 300 nm-thick silicon oxynitride film on the oxide semiconductor film, and a 300-nm silver film on the silicon oxynitride film Have

Note that in Samples 1 and 2, the oxide semiconductor film is a target for forming an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) (In: Ga: Zn = 1: 1: 0.5 [atom%]). The atmosphere for forming the oxide semiconductor film was a mixed atmosphere of argon (Ar) and oxygen (O ₂ ) (Ar: O ₂ = 30 (sccm): 15 (sccm)).

FIG. 7A shows the CV characteristics of Sample 1. FIG. FIG. 7B shows the relationship between the gate voltage (Vg) of sample 1 and (1 / C) ² . When the differential value of (1 / C) ² in the weak inversion region in FIG. 7B is substituted into Equation (3), a carrier density of 1.8 × 10 ¹² / cm ³ in the oxide semiconductor film is obtained. .

FIG. 8A shows the CV characteristics of Sample 2. FIG. 8B shows the relationship between the gate voltage (Vg) of sample 2 and (1 / C) ² . When the differential value of (1 / C) ² in the weak inversion region in FIG. 8B is substituted into Equation (3), a carrier density of 6.0 × 10 ¹⁰ / cm ³ in the oxide semiconductor film was obtained. .

  From the above results, in the MOS capacitor (sample 1) using a metal film having an effect of extracting oxygen and the MOS capacitor (sample 2) using a metal film having no effect of extracting oxygen, It was found that the carrier density was at least two orders of magnitude different. This suggested that oxygen was extracted from the oxide semiconductor film by the metal film and oxygen vacancies in the oxide semiconductor film increased, and as a result, the oxide semiconductor film in contact with the metal film became n-type. The n-type means that the number of electrons that are majority carriers increases.

[Conductivity of titanium oxide film]
In consideration of the above calculation results, the case where the metal film 70 is a metal film having a titanium crystal in the thin film transistor having the configuration shown in FIG. 1 was considered.

  At the interface between the In—Ga—Zn—O-based oxide semiconductor film (corresponding to the “oxide semiconductor film 40” in FIG. 1) and the titanium film (corresponding to the “metal film 70” in FIG. 1), titanium is present. The oxygen extracted by the reaction with titanium formed a titanium oxide film (corresponding to “metal oxide film 60” in FIG. 1). Next, the result verified by computational science about the conductivity of this titanium oxide film is shown.

  Titanium dioxide has several crystal structures such as a rutile structure (high-temperature tetragonal crystal), anatase structure (low-temperature tetragonal crystal), and brookite structure (orthorhombic crystal). The anatase type and brookite type were irreversibly changed to the most stable rutile type when heated, and therefore it was assumed that the titanium dioxide had a rutile structure.

  FIG. 9 is a diagram showing a crystal structure of titanium dioxide having a rutile structure. The rutile structure is tetragonal, and the space group showing the symmetry of the crystal belongs to P42 / mnm. In addition, the space group which shows the symmetry of a crystal also belongs to P42 / mnm similarly to the titanium dioxide of an anatase structure similarly to the titanium dioxide of a rutile structure.

  For the crystal structure of titanium dioxide, calculation for obtaining the state density was performed by a density functional method using a GGA-PBE functional. While maintaining symmetry, structural optimization including the cell structure was performed to obtain the density of states. The plane wave pseudopotential method introduced in the CASTEP code was used for the calculation using the density functional method. The cut-off energy was 380 eV.

  FIG. 10 is a state density diagram of titanium dioxide having a rutile structure. As shown in FIG. 10, it was found that titanium dioxide having a rutile structure has a band gap and has a semiconductor state density. In the density functional method, the band gap tends to be estimated to be small, and the actual band gap of titanium dioxide is about 3.0 eV, which is larger than the band gap shown in the state density diagram of FIG. Since the electronic state calculation using the density functional method is performed at absolute zero, the origin of energy is the Fermi level.

  FIG. 11 is a state density diagram of titanium dioxide having a rutile structure in an oxygen deficient state. For the calculation, a titanium oxide having 24 Ti atoms and O47 atoms obtained by extracting one O atom from a titanium oxide having 24 Ti atoms and 48 O atoms was used as a model. As shown in FIG. 11, the Fermi level in the presence of oxygen vacancies exists in the conduction band, and the density of states is not zero at the Fermi level. From this, it was found that titanium dioxide having oxygen deficiency shows n-type conductivity.

  FIG. 12 is a state density diagram of titanium monoxide (TiO). As shown in FIG. 12, it was found that titanium monoxide has a metallic density of states.

State density map of the titanium dioxide as shown in FIG. 10, the state density diagram of titanium dioxide having an oxygen deficiency shown in FIG. 11, and from the state density map of titanium monoxide shown in FIG. 12, titanium dioxide having an oxygen deficiency (TiO _{2- δ} ) was predicted to exhibit n-type conductivity over the range 0 <δ <1. Therefore, even if the composition of the titanium oxide film (metal oxide film 60) includes titanium monoxide or titanium dioxide having oxygen vacancies, the In—Ga—Zn—O-based oxide semiconductor film (oxide) The current flow between the semiconductor film 40) and the titanium film (metal film 70) is not easily inhibited.

(Embodiment 4)
The thin film transistor described in the above embodiment can be applied to a variety of electronic devices (including game machines). Examples of the electronic device include a television device (also referred to as a television or a television receiver), a monitor for a computer, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone (a mobile phone or a mobile phone device). Also, portable game machines, portable information terminals, sound reproducing devices, large game machines such as pachinko machines, solar cells, and the like. An example of an electronic device to which the thin film transistor described in the above embodiment is applied is described below with reference to FIGS.

  FIG. 13A illustrates an example of a mobile phone to which the thin film transistor described in the above embodiment is applied. This mobile phone includes a display unit 121 incorporated in a housing 120.

  This mobile phone can input information by touching the display unit 121 with a finger or the like. In addition, operations such as making a call and typing a mail can be performed by touching the display unit 121 with a finger or the like.

  For example, by arranging a plurality of thin film transistors described in the above embodiment as switching elements of pixels in the display portion 121, the performance of this mobile phone can be improved.

  FIG. 13B illustrates an example of a television device to which the thin film transistor described in the above embodiment is applied. In this television apparatus, a display portion 131 is incorporated in a housing 130.

  For example, by arranging a plurality of thin film transistors described in the above embodiment as switching elements of pixels in the display portion 131, the performance of the television device can be improved.

  As described above, the thin film transistor described in the above embodiment can be improved in performance of the electronic device by being provided in a display portion of various electronic devices.

  FIG. 14 shows a cross section of a thin film transistor using an In—Ga—Zn—O-based oxide semiconductor observed with a transmission electron microscope (TEM: Transmission Electron Microscope, Hitachi “H-9000NAR”) at an acceleration voltage of 300 kV. It is a photograph.

  In the thin film transistor illustrated in FIG. 14, an In—Ga—Zn—O-based oxide semiconductor film having a thickness of 50 nm is formed as the oxide semiconductor film 40, and then first heat treatment (650 ° C., 1 hour) is performed in a nitrogen atmosphere. Thereafter, a titanium film having a thickness of 150 nm is formed as the metal film 70, and second heat treatment (250 ° C., 1 hour) is performed in a nitrogen atmosphere.

  In FIG. 14, it was confirmed that the metal oxide film 60 was formed at the interface between the oxide semiconductor film 40 and the metal film 70. In addition, it was confirmed that the metal high concentration region 50 was formed in a region in the oxide semiconductor film 40 in contact with the metal oxide film 60. As a result of the analysis using the Fast Fourier Transform Mapping (FFTM) method, it was confirmed that crystals close to the composition of indium (In) were deposited in the metal high concentration region 50 of the thin film transistor. Similarly, it was confirmed that a titanium oxide film was formed as the metal oxide film 60.

DESCRIPTION OF SYMBOLS 10 Substrate 20 Gate electrode 30 Gate insulating film 40 Oxide semiconductor film 50 High metal concentration region 60 Metal oxide film 70 Metal film 80 Insulating film 100 region 120 Case 121 Display unit 130 Case 131 Display unit

Claims (5)

Lee indium has an oxide semiconductor film including gallium, and zinc,
The oxide semiconductor film has a first region, a second region, and a third region,
The third region is sandwiched between the first region and the second region, and has a function as a channel formation region,
A first titanium oxide film in contact with an upper surface of the first region ;
A second titanium oxide film in contact with the upper surface of the second region ;
A first titanium film having a portion overlapping with the first region across the first titanium oxide film and functioning as a source electrode;
A second titanium film that has a portion overlapping the second region across the second titanium oxide film and functions as a drain electrode;
A first titanium film, the second titanium film, and an insulating film in contact with the third region,
At the interface between the interface and the said second region second titanium oxide layer between the before and Symbol first region the first titanium oxide layer, a method for manufacturing a transistor exists grains or microcrystals of indium Because
After forming the oxide semiconductor film, a titanium film is formed in contact with the upper surface of the first region, the upper surface of the second region, and the upper surface of the third region,
Etching the titanium film to form the first titanium film and the second titanium film,
Forming the insulating film on and in contact with the first titanium film, the second titanium film, and the third region;
A method for manufacturing a transistor is characterized in that the first titanium oxide film, the second titanium oxide film, and the indium crystal grains or microcrystals are formed by performing heat treatment after the insulating film is formed. . In claim 1,
A gate electrode on the substrate;
A gate insulating film on the gate electrode;
The method for manufacturing a transistor which comprises said oxide semiconductor film on the gate insulating film. In claim 1 or claim 2,
The method for manufacturing a transistor is characterized in that the heat treatment is performed in an inert gas atmosphere or a nitrogen gas atmosphere. In any one of Claim 1 thru | or 3,
The method for manufacturing a transistor is characterized in that the heat treatment is performed at 100 ° C to 400 ° C. In any one of Claims 1 thru | or 4,
The method for manufacturing a transistor is characterized in that the insulating film is silicon oxide, silicon oxynitride, aluminum oxide, or aluminum oxynitride.